Modern digital oscilloscopes typically acquire and digitize input data representative of the behavior over time of an electrical signal. The resulting data-address pairs are typically "rasterized" to convert them into a two-dimensional bit-map for display by a raster scan display. Raster scan displays, like those in computer monitors, utilize a two-dimensional matrix of pixels arranged in rows and columns.
Although such displays can have only a single bit of intensity information associated with each pixel location, and are therefore limited to turning the pixel's intensity "on" or "off", more expensive raster scan displays use more memory depth in association with each pixel and thereby achieve a grey-scale spectrum of intensity choices for each pixel in the display.
Grey-scale raster scan displays can also provide "variable persistence", a way for the user to control how quickly or slowly the intensity of each illuminated pixel is decreased over time if no new intensity information is directed to that pixel. As new waveforms are acquired and displayed, the intensity values stored for the individual pixels at the locations associated with these waveforms are made brighter by some increment value. All illuminated pixels are also decremented on each display cycle. Pixels that are part of the waveform display of a repetitive waveform eventually attain a maximum intensity value and are displayed brightly, while pixels that are part of an intermittent feature of the waveform appear dimmer, depending on how infrequently that intermittent part of the waveform occurs and therefor how often they receive an additional increment of intensity value.
Infinite persistence refers to the oscilloscope display's behavior when illuminated pixel's intensity is not decremented at all. When a user selects this display mode, intensity data is only added to pixel values, never subtracted. In this mode, all of the pixels affected by repetitive waveforms eventually reach their maximum intensity values and stay there until the display settings are modified.
The rate at which an oscilloscope can acquire new waveforms, and therefore a limitation on often it can rasterize new data for its display, depends on something called "the trigger rate". A trigger signal indicates to the oscilloscope that some external event has met preestablished criteria defining when another data acquisition would be appropriate. In its simplest form, a trigger can be generated every time that the signal under test crosses a particular voltage level going in a particular direction. The "raw" trigger signal, a.k.a. main event trigger (MET), becomes active every time external events meet the preestablished criteria. However, for a variety of reasons, the oscilloscope may not be ready to make another acquisition yet. Raw triggers are ignored until the oscilloscope itself is again in a state of readiness to do another acquisition. A signal indicative of this readiness is typically ANDed with the MET to produce a MAT, or main accepted trigger. This is the trigger signal that controls data acquisition, since it means that both the external event and the rest of the oscilloscope are now at a suitable time for referencing the acquisition of another data record.
When the MAT occurs, the oscilloscope performs a sequence of activities to capture, or retain, data that is representative of the behavior of the signal under test. As implied above, the trigger event may initiate data capture, or terminate it, or provide a reference point somewhere in the middle of the acquired data record. Use of a circular data acquisition memory makes the time relationship between the trigger event and the time of the actual data capture highly adjustable.
Once the data associated with a particular trigger event has been acquired, however, it is still generally necessary for the instrument to perform some additional operations before it can initiate rasterization of that data. For example, in an instrument having a fast-in, slow-out (FISO) front end, data must be moved out of that front end and into a slower speed acquisition memory before the data is ready for rasterization. The time associate with this particular activity also delays the instrument's readiness to perform another acquisition of data.
While the data acquisition interval and the waveform rasterization period may vary widely with respect to each other, it is generally desirable to try to keep them happening in parallel with each other to maximize throughput. Under some circumstances, however, the rate of the MET, or external trigger, is highly variable. This can cause a variation in signal intensity that is irritating or frustrating to the user.
When digital oscilloscopes of the prior art are using their persistence mode (I.e., are not in the infinite persistence mode), they have traditionally responded to the absence of triggers by continuing to decrease the displayed waveform's intensity level until it reaches zero and the waveform fades away. If the trigger rate decreases, but some triggers continue to occur, fewer waveforms update the display and the perceived intensity fades to a level that may be difficult use. To deal with this situation, most digital oscilloscopes have a "brightness" control that allows the user to modify the value by which the intensity values of individual pixels are decremented, thereby increasing the persistence of each waveform and brightening the display. But, this too can be irritating or frustrating to the user if the trigger rate continues to vary.
If the trigger rate should increase again, after the brightness control has been adjusted to compensate for the reduced trigger rate, the waveform display will tend to go to maximum intensity in every illuminated pixel, thereby "saturating" the display. Thus, in the presence of a variable trigger rate, the user may become quite frustrated by the need to constantly adjust the brightness control in an effort to produce a display with satisfactory intensity. What is desired is some way to continue to provide a useful display in the presence of a fluctuating trigger rate. And, while this problem has been discussed in the context of multi-bit raster scan displays with persistence, an ideal solution should also work for single bit rasterizations too.